Conventional pressure diffusers, such as those sold by Kamyr, Inc. of Glens Falls, N. Y. under the trademark "MC.RTM.", are typically one stage. Some partial two stage units--attempting to gain the advantages for multi-stage units often achieved in atmospheric diffusers--are in use with no interval separation between stages, and are relatively inefficient. Attempts at full two-stage operation have been commercially unsuccessful to date.
In view of the fact that the pressure diffusers operate at a pressure that is always substantially greater than one atmosphere throughout passage of the pulp being treated through the vessel, there have been perceived impediments to multi-stage configurations. Perhaps the largest perceived impediment in conventional thinking is that there is no good way to divide the flow without resulting in channeling of the pulp, or plugging of the screens, and that mixing between the extraction flows of the different stages would occur thereby destroying the benefits of a two-stage approach.
According to the present invention, a pressure diffuser is provided which overcomes the perceived impediments to multi-stage pressure diffusers. According to the present invention a mechanism is provided which properly divides the flows without effecting channeling, or plugging, or resulting in mixing that would destroy the benefits of the multi-stage approach. In its most basic concept, this is accomplished according to the present invention by providing a baffle which extends at least the majority of the length of one of the stages, with a transverse wall adjacent the interface between the stages. This is also facilitated by preferably blanking off the screen adjacent the transverse wall, at the interface between the stages, to minimize mixing.
According to one aspect of the present invention, a pulp treating apparatus is provided comprising the following elements: A generally upright, liquid-tight, pressurized vessel defining a first interior volume for containing pulp to be treated under pressure. A pulp inlet to the vessel. A pulp outlet from the vessel, the pulp flowing generally vertically between the pulp inlet and the pulp outlet. A screen defining a surface of revolution upstanding within the vessel and defining, in part, the first interior volume containing pulp. Extraction means for withdrawing liquid from the pulp, through the screen, and including means defining a second interior volume within the vessel for receiving the extracted liquid. The extraction means comprising dividing means for dividing the second interior volume, within the surface of revolution of the screen, into at least first and second subvolumes, one located above the other; and a first extraction conduit associated with the first subvolume, and a second extraction conduit associated with the second subvolume. First treatment liquid introduction means for introducing a first treatment liquid into the first interior volume adjacent the first subvolume; and second treatment liquid introduction means vertically spaced from the first treatment liquid introduction means, and for introducing a second liquid, distinct from the first liquid, into the first interior volume adjacent the second subvolume. The first extraction conduit preferably is adjacent the top of the vessel, and the second extraction conduit adjacent the bottom, however the basic principle is equally applicable to upflow and downflow units, A blanked solid wall portion of the screen is provided at the dividing means so that extraction of liquid through the blanked screen portion is precluded. The dividing means preferably comprises a transverse plate which is disposed within a tubular baffle having a vertical axis and mounted within the screen surface of revolution, radially space d therefrom, a number of openings being provided in the tube adjacent the dividing means.
According to another aspect of the present invention, a screen and baffle assembly for a treatment vessel are provided. The assembly comprises: An annular screen elongated in a first dimension about an axis and having first and second ends. Bearing means disposed at the first end of the screen. Connection means disposed at the second end of the screen, adapted to connect the screen to a power source. An inner hollow cylinder mounted within the screen and elongated in the same dimension of elongation as the screen, and coaxial therewith and having a first end and a second end. The inner cylinder and the screen bearing means engaging at the first end of each. The cylinder having a length roughly about half of the screen length; and means defining a plurality of radially extending openings in the cylinder adjacent the second end thereof. A transverse plate closes off the interior of the cylinder adjacent--but preferably spaced from the second end of the cylinder, and the screen preferably has a blanked portion adjacent the transverse plate.
The invention also contemplates a method of treating softwood pulp (although also applicable to hardwood pulp), or like comminuted cellulosic fibrous material suspension. The method comprises the steps of: (a) Introducing the suspension under pressure into one end of the vessel so that it flows vertically within the first interior volume, the suspension being maintained at a pressure substantially greater than one atmosphere throughout its passage the vessel. (b) Extracting liquid through the screen into the second interior volume, the liquid being withdrawn from the suspension in the first interior volume. (c) Introducing a first treatment liquid through the circumferential side walls of the vessel at a first vertical portion of the vessel. (d) Introducing a second treatment liquid, distinct from the first treatment liquid, at a second vertical portion of the vessel distinct and vertically spaced from the first vertical portion of the vessel. (e) Withdrawing the majority of the extraction liquid at the first vertical portion of the vessel from the first end of the vessel; and (f) withdrawing the majority of extracted liquid at the second vertical portion of the vessel from the second end of the vessel so that minimal mixing between the extracted liquid at the first and second portions of the vessels takes place. The withdrawn liquid from step (e) is preferably provided as the introduced liquid in step (b), while the first treatment liquid is wash water, such as filtrate from a next stage washer, machine white water, screen room white water, evaporator condensate, and/or fresh wash water, depending upon location of the pressure diffuser with respect to other equipment. Preferably the screen is reciprocated up and down within the vessel.
It is the primary object of the present invention to provide multi-stage pressure diffuser, and method of pressure diffusion treatment (e.g. washing) of cellulosic pulp using such a device. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.